A solar cell converts solar energy directly to DC electric energy. Generally configured as a photodiode, a solar cell permits light to penetrate into the vicinity of metal contacts such that a generated charge carrier (electrons or holes (a lack of electrons)) may be extracted as current. And like most other diodes, photodiodes are formed by combining p-type and n-type semiconductors to form a junction.
Electrons on the p-type side of the junction within the electric field (or built-in potential) may then be attracted to the n-type region (usually doped with phosphorous) and repelled from the p-type region (usually doped with boron), whereas holes within the electric field on the n-type side of the junction may then be attracted to the p-type region and repelled from the n-type region. Generally, the n-type region and/or the p-type region can each respectively be comprised of varying levels of relative dopant concentration, often shown as n−, n+, n++, p−, p+, p++, etc. The built-in potential and thus magnitude of electric field generally depend on the level of doping between two adjacent layers.
Substantially affecting solar cell performance, carrier lifetime (recombination lifetime) is defined as the average time it takes an excess minority carrier (non-dominant current carrier in a semiconductor region) to recombine and thus become unavailable to conduct an electrical current. Likewise, diffusion length is the average distance that a charge carrier travels before it recombines. In general, although increasing dopant concentration improves conductivity, it also tends to increase recombination. Consequently, the shorter the recombination lifetime or recombination length, the closer the metal region must be to where the charge carrier was generated.
Most solar cells are generally formed on a silicon substrate doped with a first dopant (commonly boron) forming an absorber region, upon which a second counter dopant (commonly phosphorous) is diffused forming the emitter region, in order to complete the p-n junction. After the addition of passivation and antireflection coatings, metal contacts (fingers and busbar on the emitter and pads on the back of the absorber) may be added in order to extract generated charge. Emitter dopant concentration, in particular, must be optimized for both carrier collection and for contact with the metal electrodes.
In general, a low concentration of (substitutional) dopant atoms within an emitter region will result in both low recombination (thus higher solar cell efficiencies), and poor electrical contact to metal electrodes. Conversely, a high concentration of (substitutional) dopant atoms will result in both high recombination (thus reducing solar cell efficiency), and low resistance ohmic contacts to metal electrodes. Often, in order to reduce manufacturing costs, a single dopant diffusion is generally used to form an emitter, with a doping concentration selected as a compromise between low recombination and low resistance ohmic contact. Consequently, potential solar cell efficiency (the percentage of sunlight that is converted to electricity) is limited.
One solution is the use of a dual-doped or selective-emitter. A selective emitter uses a first lightly doped region optimized for low recombination, and a second heavily doped region (of the same dopant type) optimized for low resistance ohmic metal contact. However, a selective-emitter configuration may be difficult to achieve in a one-step diffusion process and may involve several masking steps, consequently increasing manufacturing costs. In addition, since there are generally no visual boundaries between high doped and low doped emitter regions, the alignment of a metal contact onto a previously deposited highly doped region may be difficult.
Like the emitter region, the deposition of a BSF (back surface field) may also be problematic. A BSF is generally a region located at the rear of a solar cell which tends to repel minority carriers in the absorber region from high recombination zones at the rear surface and metalized regions of the substrate. In general a BSF may be formed using dopants of the same type as those used in the absorber region. In this case the concentration of dopant atoms in the BSF is selected to be higher than that used to dope the absorber region, thus creating a potential barrier between the bulk of the substrate and the rear surface. A BSF is generally formed using aluminum (or other deposited materials) which is generally first screen printed onto the back of a solar cell and then co-fired in a belt furnace along with the front side metal contacts (commonly formed from screen printed silver paste). Typically, silicon atoms in the substrate tend to diffuse in the aluminum and subsequently re-crystallize, incorporating aluminum atoms into the silicon crystal.
However, although relatively easy to manufacture, the thermal expansion coefficient of aluminum (about 24 μm/m° C.) is much greater than silicon (about 3 μm/m° C.). Consequently, substrate bowing tends to occur. And while some reduction in carrier recombination is achieved with the screen printed Al BSF, there is still significant recombination occurring at the rear which tends to reduce solar cell efficiency.
In view of the foregoing, there is a desire to provide methods of using a silicon nanoparticle fluid to control in situ a dopant diffusion profile.